Building a CANDU reactor

Nuclear Reaction Review

• Uranium has a large nucleus which is barely stable

• The addition of another subatomic particle into the nucleus can force the nucleus to become unstable

• The unstable nucleus fissions (or splits) to form two new substances called daughter nuclei.

• 3 energetic neutrons are also ejected from the average reaction. These energetic neutrons are called fast neutrons

• The two daughter nuclei have less binding energy then the Uranium. This difference in energy is seen as heat energy.

Fissioning of Uranium (enriched)• 1n0 + 235U92 236U92 144Ba56 +

89Kr36 + 3 1n0 + 177 MeV

Power Plant requirements

• A method to slow energetic neutrons so they can react with Uranium nucleus

• A method to limit the number of neutrons that react in the chain reaction

• A method to absorb the heat energy to convert the heat into electricity

• Safety protocols to stop the nuclear reaction

The nuclear reaction

The nuclear reaction occurs inside the fuel channel assembly.

This assembly has to be bathed in some material to slow the energetic neutrons.

The moderator

• Moderator is the substance which slows the energetic neutrons but does not absorb them

• Heavy water is the ideal candidate

• Heavy water is a molecule in which the hydrogen contains a neutron

• The heavy water is heavier than normal (light) water

• The increase in density of the heavy water allows for more collisions with the neutrons.

• These increased number of collisions absorbs enough energy to turn the fast neutrons into slow neutrons … which can react with the uranium.

Controlling the reaction

• If the three created neutrons were allowed to react the reaction would increase exponentially– This is the model for the atomic bomb

• Ideally only one of the three neutrons would be allowed to react with the Uranium

• Two of the created neutrons must be absorbed

Control Rods

• Rods of Cadmium metal can be inserted into the reactor chamber to absorb neutrons

• If the rods are fully inserted the reaction completely shuts down

CANDU steam loop

• The heavy water moderator turns to steam as it absorbs the energy of the nuclear reaction

• This steam is pumped through a condenser loop in which thermal energy is absorbed by light water

• The light water steam is used to drive a steam turbine which converts steam into electricity

Safety• In the event of a power loss, control rods drop

into the reactor stopping the reaction• The liquid heavy water moderator can be

removed quickly … stopping the reaction

• The reactor core is a closed system. The radiated heavy water never comes into contact with the light water from the lake.

• A liquid “poison” can be added to the moderator absorbing the neutrons shutting the reaction down

• The next slide shows a schematic of a typical CANDU reactor. Use the schematic to write a short summary of the operation of the reactor.